In recent years, the intended purpose of portable devices is not limited to simple telephone calls, but includes wide range of other purposes including transmission/reception of electronic mails, connection to the Internet, and video game functions. Therefore, the development of portable devices having various mechanisms for satisfying both the portability and the operability such as large screen display and a character input function has been in progress.
For example, there is a folding-type portable device, which is usually carried in a folded state. Then, when it is used for a telephone call or mail transmission/reception, it is extended so that the display unit and the operation unit are exposed and various operations can be thereby performed. The folding-type portable device can prevent incorrect operations from being performed when it is carried. However, when it is in the folded state, the display unit is not exposed. Therefore, it is impossible to perform almost all the functions. Therefore, it is necessary to separately provide another display unit that can be viewed in the folded state.
In contrast to this, as for a slide-type portable device, the display unit is always exposed irrespective of whether the portable device is in a retracted state or in an extended state. Therefore, the display unit can be viewed in any state. By providing a minimum key operation unit at the bottom of the display unit, certain operations such as reading an electronic mail and performing an operation on the Internet can be performed, even when it is in the retracted state, i.e., when it is being carried, without changing the portable device from the retracted state. Therefore, the usability is improved. Further, when a user performs an operation such as creating a mail document, the display unit and the key operation unit are slid in parallel so that another key operation unit disposed below the liquid crystal unit is exposed and the user can operate that key operation unit (see Patent literature 1).
In contrast to this, since the full length of the folding-type portable device in the extended state becomes almost twice as long as the length in the folded state, it is possible to secure a large key operation surface. Further, it is possible to dispose the display unit at a certain angle with respect to the operation unit. Therefore, the folding-type portable device has excellent usability both when an operation is performed and when a telephone call is carried out. Further, as a technique in which this feature of the folding-type portable device is incorporated into the slide-type portable device, there are a technique that enables the display unit to be completely detached from the key operation unit (see Patent literature 2) and a technique that enables the display unit to be disposed at a certain angle with respect to the key operation unit (see Patent literatures 3, 4, 6, and 7). Further, Patent literature 5 discloses a technique that enables a portable device to have a large inclination angle after a sliding action is performed.